This invention relates to a continuous process for the separation of zirconium and hafnium tetrachlorides by distillation, wherein the tetrahalides are dissolved in a mixture of sodium and potassium chlorides to form a low-melting eutectic salt solution and the hafnium tetrachloride is removed as an overhead fraction from a thermal distillation column.
It further relates to the direct distillation of hafnium tetrachloride from a solution of hafnium and zirconium tetrachlorides in a sodium chloride-potassium chloride solvent, wherein the overhead hafnium tetrachloride vapor in a thermal distillation column is continuously washed with a reflux portion of the overhead hafnium tetrachloride in an eutectic solution of hafnium tetrachloride-sodium chloride-potassium chloride.
Still further this application relates to a process whereby the separated hafnium tetrachloride and zirconium tetrachloride are provided in the vapor phase, having sufficient purity for direct reduction to the respective metals.
In the production of zirconium and hafnium metals, the Kroll process has proved to be of great commercial interest. The Kroll process is chiefly concerned with the reduction of halides, e.g. the tetrahalides, of titanium, zirconium and hafnium with magnesium to provide the respective metals. The halides can be obtained from the metal ores by any of several well known methods, such as, for example, the formation of zirconium and hafnium carbide or carbo nitride in an arc furnace, and the subsequent chlorination thereof to mixed zirconium and hafnium tetrachlorides.
In another method, the ore is fused with a caustic soda flux and the resultant mass is water leached to separate the water soluble silicates from the water insoluble sodium zirconate and sodium hafnate. The alkali metal salts are converted to the oxides, which are directly chlorinated using carbon tetrachloride or mixtures of carbon and chlorine or to the carbide or carbo nitrides, which are then chlorinated.
While the direct reduction of the mixture of zirconium and hafnium tetrahalides is possible, and in some cases desirable for the production of alloys, such alloys find limited utility and it is more frequently desirable to produce the two metals separately in pure form. In the use of zirconium for nuclear reactors, virtually all of the hafnium must be removed due to its high thermal neutron capture cross section. Further, in the commercial production of zirconium and hafnium metals by the reduction of the respective tetrahalides, it is essential that the halides be free as possible from contamination by certain metals, such as iron and aluminum. It is also extremely important to remove gaseous impurities, such as oxygen and nitrogen present in any form, and to prevent further contact of the halide therewith, since oxygen and nitrogen will be carried into the finished metal causing it to be hard and brittle and thus difficult to work and not meeting established specifications.
The common commercial techniques for the separation of hafnium from zirconium are accomplished in extremely complex liquid-liquid extraction processes. In these processes the crude tetrachloride, consisting of chemically mixed zirconium tetrachloride and hafnium tetrachloride plus impurity chlorides, is dissolved in an aqueous solution containing a complexing agent. This aqueous solution is then contacted with a selective solvent, which serves to extract one of the elements. Both the extract and the raffinate are then treated to recover and recycle the chemicals used in the solution and in the liquid-liquid extraction, and the metal salts are then precipitated, filtered, washed, calcined, and rechlorinated. The equipment and processing entailed encompass a large portion of the entire processing facility for the production of zirconium and hafnium metals. The processes are complex, difficult to control, and utilize considerable amounts of relatively expensive and corrosive chemicals resulting in the use of expensive and short-lived equipment. In addition considerable amounts of labor are expended in the complex manipulation required. The separatory technique is a major contributor to the manufacturing cost of zirconium and hafnium metals. The separated tetrahalides of zirconium and hafnium must usually be treated subsequent to the separatory processing to remove the oxygen and nitrogen containing compounds, as well as iron and aluminum.